1. Field of the Invention
The present invention relates to printers such as those associated with computers and facsimile systems, laser printers, LED printers and liquid-crystal printers. More particularly, the present invention pertains to electrophotographic printers of the type in which a visible image produced by a developer in accordance with image data is formed on a recording material by an electrophotographic method.
2. Description of the Related Art
The above-described electrophotographic method has originally been adopted in copying machines. However, electrophotographic systems have recently been widely employed as image output devices, i.e., printers, because they feature high printing quality, high-speed printing and low noise level.
In electrophotographic printers which employ a toner, i.e., colored fine particles, as a developer, after a visible image of toner formed on a photosensitive member has been transferred to a recording material, the toner remaining on the photosensitive member must be removed by a cleaner. In such case, the toner removed from the photosensitive member by the cleaner has heretofore been discarded without being reused, uneconomically.
In order to overcome such disadvantage, one type of electrophotographic printer has already been developed which enables the toner removed from the photosensitive member by the cleaner to be reused.
Such conventional electrophotographic printer, which enables the toner to be reused, will first be described with reference to FIG. 2.
An electrophotographic printer of the type described above has a housing 1 in which a photosensitive drum 2 as one example of the photosensitive member is disposed in such a manner that the drum 2 is rotatable in the direction of the arrow. A charger 3 is disposed within the housing 1 and to one side of the photosensitive drum 2, the charger 3 being adapted to accumulate electrostatic charges on the surface of the drum 2. This charger 3 also serves as a transfer device which transfers a visible image of toner to a sheet of recording paper as one example of the recording material. An optical writing member 4 is disposed within the housing 1 and above the photosensitive drum 2, the member 4 having a laser head 5 for forming an electrostatic latent image on the drum 2 charged by the chager 3. A developing device 6 which also serves as cleaner is disposed within the housing 1 and to the other side of the photosensitive drum 2. The developing device 6 has a casing 7, and a blade 8 is disposed in the upper part of the casing 7, the blade 8 being brought into contact with the photosensitive drum 2 so as to function as a cleaner. A developing sleeve 9 is disposed in the lower part of the casing 7, the sleeve 9 being adapted to attach a toner T filled within the casing 7 to the photosensitive drum 2. In addition, a charge remover 10 for removing any charge remaining on the photosensitive drum 2 is disposed within the housing 1 and below the drum 2.
A hopping roller 12 is disposed in the lower part of the housing 1 for supplying sheets of recording paper in a paper feed cassette 11 one by one. A transport passage 13 having transport rollers 14 is defined inside the housing 1 so that each sheet of recording paper supplied from the paper feed cassette 1 is passed through the area defined between the photosensitive drum 2 and the charger 3. In addition, a fixing device 15 having a pair of fixing rollers 16 and a pair of transport rollers 17 is disposed at the downstream side of the transfer passage 13 so that a visible image of toner adhering to the surface of the recording paper is fixed thereto by heat and pressure. The recording paper which has passed the fixing device 15 is discharged onto a deliver tray 18 which is provided in the housing 1 in such a manner as to project therefrom, the recording paper being delivered in a face-up state, i.e., in a state wherein the printed surface of the paper is directed upward.
With the above-described arrangement, the photosensitive drum 2 is charged at a predetermined potential by means of corona discharge caused by the charger 3 during a first turn of the drum 2, and an electrostatic latent image is formed on the surface of the drum 2 by the optical writing member 4 through exposure in accordance with particular image data. Thereafter, a toner T uniformly charged by friction or injection of electric charges is transported within the casing 7 to a position which faces the photosensitive drum 2 by means of the developing sleeve 9, and the toner T is selectively attracted to the surface of the drum 2 so as to adhere thereto in correspondence with the latent image, thereby forming a visible image. Then, the uppermost one of the sheets of recording paper in the paper feed cassette 11 is transported to the area between the photosensitive drum 2 and the developing device 6 serving also as a transfer device by the action of hopping roller 12 and the transport rollers 14, and corona discharge is caused by the developing device 6 from the rear side of the recording paper during a second turn of the photosensitive drum 2, thereby transferring the visible image of toner T on the drum 2 to the recording paper. The transferred visible image is fixed to the recording paper by the operation of the fixing device 15 to obtain a final image.
The residual surface potential on the unexposed area of the photosensitive drum 2 is optically made zero by the charge remover 10. On the other hand, the toner T which has not been transferred and remains on the photosensitive drum 2 is removed from the drum 2 by the blade 8 and dropped into the casing 7 so as to be reused, thus completing one printing process.
The above-described conventional electrophotographic printer, shown in FIG. 2, suffers, however, from the following problems. Due to the positional relationship between the photosensitive drum 2 and the optical writing member 4, the drum 2 needs to be rotated two turns in order to complete one printing process, which means that the printing efficiency is unfavorably low.
Further, since the electrophotographic printer shown in FIG. 2 discharges recording paper in a face-up state, when information which needs a plurality of sheets of recording paper is printed in a paginal sequence, one sheet of recording paper which should be placed under another is laid on top of the latter, which means that the stacked sheets of recording paper must be rearranged after printing.
In addition, since the fixing device 15 is disposed at the substantially central position inside the housing 1, special consideration is needed to eliminate adverse effects of the heat generated in the fixing device 15 on various devices and members such as the photosensitive drum 2.